userfaultfd: non-cooperative: report all available features to userland

This will allow userland to probe all features available in the kernel.
It will however only enable the requested features in the open userfaultfd
context.

Link: http://lkml.kernel.org/r/20161216144821.5183-8-aarcange@redhat.com
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Michael Rapoport <RAPOPORT@il.ibm.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/userfaultfd.c

@@ -1285,28 +1285,31 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx,
 	struct uffdio_api uffdio_api;
 	void __user *buf = (void __user *)arg;
 	int ret;
+	__u64 features;
 
 	ret = -EINVAL;
 	if (ctx->state != UFFD_STATE_WAIT_API)
 		goto out;
 	ret = -EFAULT;
 	if (copy_from_user(&uffdio_api, buf, sizeof(uffdio_api)))
 		goto out;
-	if (uffdio_api.api != UFFD_API ||
-	    (uffdio_api.features & ~UFFD_API_FEATURES)) {
+	features = uffdio_api.features;
+	if (uffdio_api.api != UFFD_API || (features & ~UFFD_API_FEATURES)) {
 		memset(&uffdio_api, 0, sizeof(uffdio_api));
 		if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
 			goto out;
 		ret = -EINVAL;
 		goto out;
 	}
-	uffdio_api.features &= UFFD_API_FEATURES;
+	/* report all available features and ioctls to userland */
+	uffdio_api.features = UFFD_API_FEATURES;
 	uffdio_api.ioctls = UFFD_API_IOCTLS;
 	ret = -EFAULT;
 	if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
 		goto out;
 	ctx->state = UFFD_STATE_RUNNING;
-	ctx->features = uffd_ctx_features(uffdio_api.features);
+	/* only enable the requested features for this uffd context */
+	ctx->features = uffd_ctx_features(features);
 	ret = 0;
 out:
 	return ret;